With the gradual depletion of subterranean and shallow subsea hydrocarbon reservoirs, the search for additional petroleum reserves is being extended to deeper and deeper waters on the outer continental shelves of the world. As such deeper reservoirs are discovered, increasingly complex and sophisticated production systems have been developed. It is projected that offshore exploration and production facilities will soon be required for probing depths of 6,000 feet or more. Since bottom founded structures are generally limited to water depths of no more than about 1,500 feet by current technology and because of the sheer size of the structure required, other so-called compliant strutures have been developed.
One type of compliant structure receiving considerable attention is a tension leg platform. (TLP). A TLP generally comprises a semisubmersible-type floating platform anchored by piled foundations through vertically oriented members or mooring elements called tension legs. The tension legs are maintained in tension at all times by insuring that the buoyancy of the TLP exceeds its operating weight under all environmental conditions. The TLP is compliantly restrained in the lateral directions allowing limited sway, surge and yaw while vertical plane movements of heave, pitch and roll are stiffly restrained by the tension legs.
Current TLP designs utilize heavy walled steel tubulars for the mooring elements. These tension legs constitute a significant weight with respect to the floating platform, a weight which must be overcome by the buoyancy of the floating structure. For instance, the tension legs utilized on the world's first commercial TLP which was installed in the Hutton Field of the United Kingdom North Sea in 485 feet of water comprised steel tubulars have an outer diameter of 10.5 inches and an inner bore of 3.0 inches. It should be readily apparent that, with increasingly long mooring elements being required for a tension leg platform in deeper and deeper waters, floating structures having the necessary buoyancy to support the extra weight of such mooring elements must be increasingly larger and more costly. Further, the handling equipment for installing and retrieving the long, heavy tension legs adds excessive weight and complexity to a tension leg platform system. Flotation systems can be utilized but are generally very costly and their reliability is questionable. In addition, the increased size of a flotation module can result in an increase in the hydrodynamic forces on the structure.
In an effort to lower the weight of deep water tension legs while retaining the strength of the heavy steel tubulars, it has been proposed that high modulus composite structures of carbon fiber be employed.
Another means for reducing the weight and complexity of a TLP, particularly in deep water applications would be to simplify and lighten the well riser system. The use of tubular steel risers for deep water development requires complicated and expensive tensioning and motion compensation equipment. In order to maintain compatable riser profiles for the prevention of hydrodynamic interaction and contact between a multiplicity of adjacent risers, the top tension applied to each of the risers must be significantly greater than the riser weight. This not only requires the use of expensive tensioning equipment as the weight of the risers increases with increasing depth, it also leads to an increase in the required platform displacement.
One approach for reducing the apparent weight of the risers would be to add syntactic foam floatation modules. However, flotation modules are costly and markedly increase the overall diameter of the riser assembly resulting in larger hydrodynamic forces which must be compensated for in the tensioning and motion compensation system. Furthermore, flotation modules require large distances to be provided between adjacent risers to prevent interaction thereby leading to an increase in the required well deck area of the platform.
Steel risers require an expensive and complicated motion, compensation system to reduce the fluctuating riser loads due to wave action and platform movement. Such apparatus must also compensate for temperature and pressure effects which reduce the pretension on the riser in use.